1. Field of the Invention
The present invention relates to fluorescent spectroscopy devices and to methods for quantifying the concentration of components in a mixture of petroleum products, and particularly to an apparatus and method for measuring concentrations of fuel mixtures using depth-resolved laser-induced fluorescence.
2. Description of the Related Art
Fluorescent spectroscopy is a tool that has been used for the qualitative and quantitative analysis of compounds that exhibit the phenomena of fluorescence and phosphorescence. When molecules are irradiated by energy of a particular frequency or wavelength, the electrons experience a transition from the ground state to an excited state due to the absorbance of photons. The electrons return to the ground state by any of several different routes known as deactivation processes. The preferred route is the path that provides the shortest lifetime in the excited state. For certain compounds under appropriate conditions, fluorescence is the preferred deactivation process. Generally, a molecule excited at an absorption frequency will exhibit fluorescence at a lower frequency—longer wavelength emission band. Compounds exhibiting fluorescence usually contain an aromatic functional group or highly conjugated double bond structures, with the intensity increasing with the number of condensed rings per molecule.
Fluorescent spectroscopy takes advantage of these properties. FIG. 14 shows a block diagram of a conventional fluorometer 100 or spectrofluorometer. The fluorometer contains a light source 102, such as a xenon lamp, capable of emitting ultraviolet light (UV). A portion of the light emitted by light source 102 passes through a first monochromator 104 into the sample 106, which is usually contained in a cuvette made from quartz, fused silica, or other material that has a high transmittance to UV radiation. Another portion of the light emitted by light source 102 passes through an attenuator 108 to a reference photomultiplier tube 110, which generates a voltage that provides one input to a differential amplifier 112 or other detector.
The sample 106 emits fluorescent light when transitioning from the excited state to the ground state. The emitted fluorescent light passes to a second monochromator 114 and a sample photomultiplier tube 116, which provides a second input voltage to the differential amplifier 112. The output of the differential amplifier 112 is fed to an analog meter 118, digital readout, plotter or chart recorder, or other output device, which displays the intensity of the fluorescent radiation.
The monochromators include an entrance slit, usually of variable width, and various slits, lenses, mirrors, windows, and a beam dispersal device, usually either a prism or a grating. The monochromator filters or narrows the received light beam to a single frequency or wavelength of interest at a time, and provides for continuously changing the wavelength, usually by rotating the beam dispersal device, the latter process being termed “scanning” the spectrum. Usually the second monochromator is positioned at 90° to the incident light beam from light source 102 in order to minimize the effects of scattering.
In recent years, laser light sources have become available as an alternative to the conventional ultraviolet lamp. Early lasers were limited to a few discrete wavelengths, but dye pulse lasers allow for continuous variation of the wavelength, so that the first monochromator 104 is unnecessary when the light source is a pulsed laser. Conventional fluorometers may be provided with a sample holder turntable that can accommodate more than one cuvette, with the turntable being rotated to place each cuvette successively in the path of the beam from the light source.
The composition of a mixture of fluorescent substances can be analyzed with a conventional spectrofluorometer in the following manner. For each individual component known to be in the mixture, the emission wavelength band is scanned with the excitation wavelength fixed to find maximum intensity. Then, with the emission wavelength fixed at the maximum intensity, the excitation spectrum is scanned for maximum and minimum intensities. The emission spectrum for each of these excitation wavelengths is scanned, and an optimal excitation-emission wavelength pair is selected for that component. Excitation and emission spectra are obtained for the wavelength pairs so selected. For each component, a concentration calibration curve is made from solutions of known concentration at each of the optimal wavelength pairs, which should be linear. The intensity of the unknown mixture is determined at each optimal excitation-emission pair, and the corresponding concentrations of the components in the mixtures can then be determined from the calibration curves.
Fluorescent spectroscopy is particularly useful, when available, due to the sensitivity of detection and the linearity of fluorescent intensity with concentration.
For many reasons, it is necessary to test petroleum products to determine purity and quality. For example, in some areas the more expensive petroleum fuels may be diluted with less expensive petroleum fuels, either intentionally to deceive the purchaser, or unintentionally as the result of contamination in the refining or storage and transport process. While some methods are available for particular analyses, e.g., the separation and quantification of mixtures of fuels having different octane numbers by gas chromatography, such methods are expensive, time consuming, and complicated.
Petroleum products are known to exhibit fluorescence. However, petroleum fuels, such as kerosene, gasoline, and diesel fuel, are each composed of mixtures of different hydrocarbons that fall within certain boiling point ranges loosely coordinated with molecular weight ranges. The type and distribution of hydrocarbons within each class of fuel may also vary according to the geographical source of the crude oil and the type of refining method (distillation, cracking, etc.). Petroleum products are dense, contain mixtures of hydrocarbons having overlapping excitation-emission spectra so that fluorescent emissions may be reabsorbed, and also may be contaminated with quenching compounds. For these reasons, fluorescent spectroscopy is not widely used in the industry.
Nevertheless, several efforts have been made to apply spectrofluorometric methods for quantitative and qualitative analysis of petroleum products. Patra and Mishra report the use of synchronous fluorescent scan spectroscopy, in which both excitation and emission monochromators are scanned simultaneously while keeping a fixed wavelength interval between them, to analyze mixtures of petrol, diesel and kerosene in The Analyst, Vol. 125, pp. 1383-1386 (2000). Patra and Mishra also report a technique using a 3-dimensional emission/excitation intensity contour diagram or matrix and the subtraction of spectral volumes to evaluate the adulteration of petrol by kerosene in Applied Spectroscopy, Volume 55, Number 3, pp. 338-342 (2201). Hidrovo and Hart describe a technique for measuring the thickness of an oil film utilizing the reabsorption and emission of two fluorescent dyes by emission reabsorption laser induced fluorescence in Measurement Science and Technology, Vol. 12, pp. 467-477 (2001).
However, none of the above apparatus and methods, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, an apparatus and method for measuring concentrations of fuel mixtures using depth-resolved laser-induced fluorescence solving the aforementioned problems is desired.